1. Field of the Invention
The present invention pertains to private automatic branch exchange telephone systems and more particularly to such systems employing time division multiplex (TDM) switching of digital signals with included provisions for interfacing a fixed format of a source such as a T1 carrier span line with the random format of an exchange switching system.
2. Description of the Prior Art
Private automatic branch telephone exchanges function as centralized switching systems. Such systems provide connections between a number of locally connected telephone lines with associated telephone apparatus and one or more trunk circuits connecting the private automatic branch exchange to a distant central office.
Until very recent times private automatic branch exchanges (PABX's) have provided switching between lines and trunks on a space divided basis. That is to say, switches of either an electro-mechanical or electronic configuration have provided selective paths through the switching system to interconnect lines to each other or to trunk circuits serving the PABX. Generally, the signals transmitted through the PABX were of an analog nature. Where a line or a trunk circuit was employed digital techniques such as pulse code modulation (PCM) were employed. Interface circuitry was a necessity. Such circuitry provided analog to digital conversion and digital to analog conversion.
More recently a new generation of PABX systems employing time division switching have been provided. Some such systems as the "Dimension" PABX manufactured by Western Electric Company have provided time division switching of analog signals. Other more recent developments in PABX systems have provided time division switching of pulse code modulated signals. Systems of this type have been manufactured by GTE Automatic Electric Company and designated GTD 120, GTD 1000 and GTD 4600. In such systems as the GTD series, analog to digital and digital to analog interfaces have been provided between the lines and trunks and the time division switching system.
To effect greater economies in transmission equipment requirements, more extensive use has been made in recent years of digital transmission equipment. Of particular wide acceptance has been the so called T1 type carrier systems which employ pulse code modulation to provide a number of multiplexed signal paths over a single transmission facility. Such arrangements are currently in use primarily between telephone central offices. To date, little utilization of such economies has taken place in transmission facilities which connect central offices and private automatic branch exchanges. The state of the art and time division switching systems, employing pulse code modulated signals as the transmission format, are exemplified by such systems as the aforementioned GTD 120 the operation of which is described in U.S. Pat. No. 4,007,338 issued to D. W. McLaughlin on Feb. 8, 1977. The use of T1 lines using signalling of the D2 or D3 PCM type is discussed in an article "Second Generation Toll Quality PCM Carrier Terminal" by L. Dean Crawford in the April 1972 issue of the Automatic Electric Technical Journal. A channel bank unit of the type employed in usage as described above is manufactured by GTE Lenkurt Incorporated and designated 9002A channel bank.
Accordingly, it is the object of the present invention to provide switching facilities to directly interface conventional T1 carrier span lines with a fixed format channel allocation. These facilities are located in a private automatic branch exchange with an associated random allocation channel network.